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Plastic bag pistons?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Plastic bag pistons?

Unread postAuthor: tghhs » Wed Apr 01, 2009 5:28 am

Hey guys not sure if this should be here as it's got some different stuff in it and kinda a running a update, so moderators feel free to move.

ok guys and girls here's my latest cannon, i've made a few before but this is probaly the only one worth posting (even though it's not done).

its just a big coaxial

the last two pics are of the chamber which is an old sign post which is 50mm and the barral which is 15mm, you can also see the end-cap to hold the pilot and shrader side on.

Your opinion of the first pic is what im most interested in. the left piston was made out of shopping bags like this:
http://www.instructables.com/id/HomemadePlastic/
and then i added a coat of silicon to the out side for a smother fit.
the middle is a old passion fruit can with neoprene around the edge.
the right is a 50mm cap for scale.

has any one tryed these as pistons and did they work? do u reckon it would?

any other comments are also most welcome :)
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Attachments
01042009(006).jpg
chamber & barral
01042009(004).jpg
close up of cap
01042009(005).jpg
pistons
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Unread postAuthor: CasinoVanart » Wed Apr 01, 2009 7:14 am

Shopping bag pistons are cool.
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Unread postAuthor: Carlman » Wed Apr 01, 2009 7:15 am

CasinoVanart wrote:Shopping bag pistons are cool.


hippy

:roll:





:D
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Unread postAuthor: CasinoVanart » Wed Apr 01, 2009 7:17 am

What.....hey who said that.

was it the dude that cant get a hybrid to work?

:high:
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Unread postAuthor: psycix » Wed Apr 01, 2009 7:23 am

That wood construction... is that to have a servicable cap?
Is it still airtight?
Beware, because if it pops off, its going to hit hard. I'd say you screw in a bolt through the pipe and cap to lock it down.

That piston method is nice. Its cool that you tried it.
How sturdy is the piston? And how heavy is it?
Nice job!
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 01, 2009 7:38 am

CasinoVanart wrote:I like pie


munchies eh?

Love the original endcap attachment - see psycix, no need for threaded fittings :P :D
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Unread postAuthor: CasinoVanart » Wed Apr 01, 2009 7:40 am

Yo dude peeps are sayin that green gas is just propane, thier wrong, it has silicone in it!!! :D :roll:
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Unread postAuthor: suburban spudgunner » Wed Apr 01, 2009 8:26 am

Dunno if anyone's tried a shopping bag piston before. I'd like to see how well it works.
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Unread postAuthor: SEAKING9006 » Wed Apr 01, 2009 9:16 am

How many bags it take? It took me all afternoon to cover the bottom of a soda can.
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Unread postAuthor: Biopyro » Wed Apr 01, 2009 9:40 am

You can make even stronger platic from old HDPE milk jugs. Cut it into flat layers, then put in a dry pan and heat until it melts at around 150*C. Try not to overheat it too much as it does smell. The plastic is very strong, but because it solidifies at such a high temperature, it's hard to mould. From what I can tell it will make great ammo, but it's hard to get a tight fit when you're forcing it into a pipe wearing wooly gloves to protect your hands.
It has other plus sides that it is less dense than water, has low friction and sticks to nothing at all, so if you use a fish splice to scrape it off the pan, it doesn't stick at all!
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Unread postAuthor: Carlman » Wed Apr 01, 2009 9:59 am

CasinoVanart wrote:What.....hey who said that.

was it the dude that cant get a hybrid to work?

:high:


scuse me it is operational thank you very much
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 01, 2009 10:10 am

Biopyro wrote:You can make even stronger platic from old HDPE milk jugs. Cut it into flat layers, then put in a dry pan and heat until it melts at around 150*C. Try not to overheat it too much as it does smell. The plastic is very strong, but because it solidifies at such a high temperature, it's hard to mould. From what I can tell it will make great ammo, but it's hard to get a tight fit when you're forcing it into a pipe wearing wooly gloves to protect your hands.
It has other plus sides that it is less dense than water, has low friction and sticks to nothing at all, so if you use a fish splice to scrape it off the pan, it doesn't stick at all!


Epoxy, epoxy, epoxy.
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Unread postAuthor: mark.f » Wed Apr 01, 2009 10:14 am

Question: why not get rid of the wood which is just making the joint weaker, and thread some machine screws (bolts) equally spaced around the endcap into the pipe, coated with teflon tape. The endcap, as you and MrCrowley have figured out, has a tapered socket, which seals well around the pipe, so just use the screws to hold the cap on. No need to complicate things with the whole wooden "cap-holder jig".

Also, you can usually find PVC fittings the same diameter as the chamber, so those would make wonderful pistons.
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Unread postAuthor: Biopyro » Wed Apr 01, 2009 10:22 am

jackssmirkingrevenge wrote:Epoxy, epoxy, epoxy.

I know how much you love it, but when I've used epoxy glue, it's often quite brittle. Admittedly this is cheap epoxy and in thin layers, but is it like this when scaled up too?

I have no doubt at all that a HDPE bullet would survive impact with anything when fired from a non hybrid, but would epoxy?
Also, epoxy sticks to PVC when casting doesn't it? It has a higher friction coefficient and density. On the plus side, it is about 100x easier to work with.

Is epoxy putty much different from the cheap glue?
(Also HDPE is free!)
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Unread postAuthor: psycix » Wed Apr 01, 2009 11:24 am

epoxyman wrote:Love the original endcap attachment - see psycix, no need for threaded fittings
He DID use bolts to secure it though... :roll:
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